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Forced Rossby Waves in Observed Background Flows

J. EggerMeteorologisches Institut, Universität München, Federal Republic of Germany

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Abstract

We consider forced quasi-stationary Rossby waves in background flows with realistic variability in time and space. Long-term integrations of the nonlinear barotropic vorticity equation for deviations from background flow are carried out where a forcing term provides sources and sinks of vorticity for the deviations. The background flow is specified according to daily flow observations at 300 hPa. Mean deviation patterns for individual months and for the total period on record (18 months) are computed and compared to the results of more conventional linear and nonlinear computations where a zonal background flow is essentially fixed in time. It is the purpose of this paper to estimate the impact of the variability of the background flow in computations of quasi-stationary Rossby waves. For how long must a forcing act to make the application of linear theories to the response promising? Results are sensitive to the choice of the surface friction parameter C and eddy diffusivity D. For C = 1/(10 days), D = 1 × 105 m2 s−1 there are considerable discrepancies of computed monthly means and the corresponding solutions for fixed background flow. There is good agreement, however, if averages over the complete period of integration are compared. This suggests that conventional linear theories can be applied to climatological stationary waves. The variability of the background flow cannot be neglected, however, if events of only a few months duration are considered.

Abstract

We consider forced quasi-stationary Rossby waves in background flows with realistic variability in time and space. Long-term integrations of the nonlinear barotropic vorticity equation for deviations from background flow are carried out where a forcing term provides sources and sinks of vorticity for the deviations. The background flow is specified according to daily flow observations at 300 hPa. Mean deviation patterns for individual months and for the total period on record (18 months) are computed and compared to the results of more conventional linear and nonlinear computations where a zonal background flow is essentially fixed in time. It is the purpose of this paper to estimate the impact of the variability of the background flow in computations of quasi-stationary Rossby waves. For how long must a forcing act to make the application of linear theories to the response promising? Results are sensitive to the choice of the surface friction parameter C and eddy diffusivity D. For C = 1/(10 days), D = 1 × 105 m2 s−1 there are considerable discrepancies of computed monthly means and the corresponding solutions for fixed background flow. There is good agreement, however, if averages over the complete period of integration are compared. This suggests that conventional linear theories can be applied to climatological stationary waves. The variability of the background flow cannot be neglected, however, if events of only a few months duration are considered.

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